Coextruded films with one side heat-sealable serve a growing market as lidding films for thermoformed trays and blister packs used in packaging. A common thermoforming material for such trays, in particular blister pack trays, is amorphous polyester sheet (APET or PETG). The term APET refers to PET (polyethylene terephthalate) sheet that has been extruded and quenched on a cooling drum or a water bath before it has time to crystallize and thus maintains its transparency. Transparency is further maintained during the thermoforming step, which is conducted at a reheat temperature above PET's glass transition temperature (around 80° C.), which is high enough to soften the material but not so high that it would lead to chain rearrangement resulting in formation of crystal domains which produce internal haze. The term PETG refers to an amorphous (non-crystallizable) PET copolyester modified with a glycol (1,4-cyclohexyldimethanol).
A sealable lidding film can be used to cover such trays. A PET-based lidding film is an extruded biaxially oriented PET, which, unlike the underlying APET container, has been subjected after the extrusion step (which is similar to the extrusion step used to produce the APET sheet) to an additional treatment consisting of rapid stretching at elevated temperatures along the machine direction, followed by stretching along the transverse direction, with heat-setting steps after each stretching or orientation step. This rapid stretching is effective in introducing crystallinity without compromising film clarity.
One important physical property for lidding films used in blister packs is easy slipping as an aid for film handling and machinability during winding, unwinding, and moving through various sections of the package forming and handling processing, typically characterized by coefficient of friction (COF). Typical coefficient of friction for polyester films is 0.4-0.5, which can be modified or controlled to some extent by manipulating the surface roughness of the film's surface. One method is by adding inorganic particles (i.e. anti-blocking additives) in the film-making process. Another method may be by adding low molecular weight lubricants (slip additives) such as low or high viscosity silicone oils, fatty amides, waxes, or combinations thereof. Such methods to control COF are not limited by these examples, however.
In more detail, unmodified film (i.e., film with high or uncontrolled COF) can be easily scratched during handling, can be prone to wrinkling and web breaks during winding, slitting, and other handling, and thus can be limited in terms of processing speed, material efficiency, and productivity. Adding inorganic or crosslinked particles into the film makes a rougher surface, which results in a lower friction coefficient. An additional requirement once a package is formed, is easy stick-free movement through the packaging and dispensing process, such as vending machines.
When coefficients of friction lower than those typically accomplished by particles are desired (e.g., COF of 0.3-0.4), the conventional solution is the use of slip agents, typically coatings or additives such as waxes or silicones. However, a disadvantage of coatings is wear debris or build-up on machinery surfaces as the packages move through automated packaging equipment, which accumulate over time, causing stoppages and/or requiring cleaning. Another disadvantage is that when coatings are applied to a heat-sealable surface, they are usually detrimental to the heat-sealing characteristics.
Alternatively, one could envision increasing the loading of inorganic particles above the typical addition level; this, however, can result in a loss of heat-sealing performance as well.
A need exists therefore for lidding films having a heat-sealable surface with slip characteristics, which maintain their heat-sealability after being modified for low coefficient of friction.